1. Field of the Invention
The present invention relates to a differential capacitor, a differential antenna element, and a differential resonator. More particularly, the present invention relates to a differential capacitor, a differential antenna element, and a differential resonator formed on a semiconductor substrate.
2. Description of the Background Art
In recent years, more miniaturized and higher-frequency oriented semiconductor processes have been proposed, as a result of which it has become commonplace to integrate a differential oscillation circuit on a semiconductor. FIG. 17 is a schematic diagram illustrating the structure of a commonly-used differential oscillation circuit 7. In FIG. 17, the differential oscillation circuit 7 comprises: first and second transistors 1001 and 1002 for enabling oscillation; first and second capacitors 1003 and 1004 for composing a differential capacitor and cutting off a DC component; first and second variable capacitors 1005 and 1006 for enabling resonance; first and second inductances 1007 and 1008 for enabling resonance; and a constant current source 1009.
Via a Vcc terminal, a DC current is supplied to the first and second inductances 1007 and 1008 of the differential oscillation circuit 7. The DC current is supplied to the first and second transistors 1001 and 1002, and thereafter flows to ground via the constant current source 1009. The first transistor 1001 is connected so as to realize positive feedback, and generates either one of an in-phase signal or a reverse-phase signal composing a differential pair of signals, having an oscillation frequency which depends on circuit constants of the first variable capacitor 1005 and the first inductance 1007. The second transistor 1002, which operates similarly to the first transistor 1001, generates the other one of the in-phase signal or the reverse-phase signal. The generated in-phase signal and reverse-phase signal are output from the Vo+terminal and the Vo−terminal, respectively, or vice versa.
However, in the above-described differential oscillation circuit 7, the influence of a parasitic capacitance Cpa and a parasitic resistance Rpa (shown as dot-line images in FIG. 17) associated with the first capacitor 1003 increases as the oscillation frequency is increased. As a result, the sharpness of the resonance caused by the first variable capacitor 1005 and the first inductance 1007 is deteriorated; that is, a quality factor (which serves as an index of resonance sharpness) is deteriorated. The sharpness of the resonance caused by the second variable capacitor 1006 and the second inductance 1008 also suffers a similar deterioration.
FIG. 18A is an upper plan view illustrating the structure of a differential capacitor (the first capacitor 1003 and the second capacitor 1004) which is integrated on a semiconductor substrate. FIG. 18B is a cross-sectional view showing the first capacitor 1003 and the second capacitor 1004, taken along a vertical plane A-A′ shown in FIG. 18A. In FIGS. 18A and 18B, the first capacitor 1003 and the second capacitor 1004 are to be formed within an interlayer film 1019 on a semiconductor substrate 1020, which is typically composed of p-type silicon. The interlayer film 1019 is typically composed of silicon oxide. More specifically, the first capacitor 1003 includes an upper electrode 1015 and a lower electrode 1016, composed of metal wires which are typically aluminum. The upper electrode 1015 and the lower electrode 1016 are disposed parallel to each other, with a predetermined interval along the vertical direction. The second capacitor 1004 is composed of the same material as the first capacitor 1003, and includes an upper electrode 1017 and a lower electrode 1018, which are formed in symmetrical positions from the upper electrode 1015 and the lower electrode 1016, respectively, with respect to the predetermined vertical plane B-B′.
The first capacitor 1003 and the second capacitor 1004 suffer from the aforementioned parasitic capacitances, which occur in the interlayer film 1019 between the semiconductor substrate 1020 and the lower electrodes 1016 and 1018, respectively. Furthermore, the aforementioned parasitic resistances also occur in the semiconductor substrate 1020. Among these parasitic components, the parasitic resistances occurring in the semiconductor substrate 1020 affect the quality factors of the resonator circuits in particular.
In order to suppress such parasitic resistances, a differential capacitor (hereinafter referred to as the “conventional differential capacitor”) as follows has been proposed. FIG. 19A is an upper plan view schematically showing the structure of the conventional differential capacitor. FIG. 19B is a cross-sectional view showing the first capacitor 1003 and the second capacitor 1004, taken along a vertical plane A-A′ shown in FIG. 19A. The conventional differential capacitor shown in FIGS. 19A and 19B differs from that shown in FIGS. 18A and 18B in that a shield plate 1021 is additionally comprised. Otherwise, the two differential capacitors are identical. Therefore, in FIGS. 19A and 19B, those component elements which have their counterparts in FIGS. 18A and 18B are denoted by the same reference numerals as those used therein, and the descriptions thereof are omitted.
The shield plate 1021, which is a plate-like structural component composed of a conductive material such as aluminum, is disposed between the semiconductor substrate 1020 and the lower electrodes 1016 and 1018. More specifically, the shield plate 1021 has a shape such that, when the lower electrodes 1016 and 1018 are projected onto the shield plate 1021 from vertically above, the projected lower electrodes 1016 and 1018 appear as being contained within the outer contour of the shield plate 1021. The shield plate 1021 has a symmetrical shape with respect to the aforementioned vertical plane B-B′. With the shield plate 1021 as such, the parasitic capacitances in the interlayer film 1019 are increased, but the parasitic resistances in the semiconductor substrate 1020 are reduced.
By applying the conventional differential capacitor structure to the differential oscillation circuit 7 as shown in FIG. 17, the sharpness of the resonance caused by each resonator circuit can be improved. Specifically, as shown by an equivalent circuit of FIG. 20, the following parasitic components will appear in the differential oscillation circuit 7: a parasitic capacitance Cpa1 between the lower electrode 1016 and the shield plate 1021, a parasitic capacitance Cpa2 between the lower electrode 1018 and the shield plate 1021, a parasitic capacitance Cpc between the shield plate 1021 and the semiconductor substrate 1020, and a parasitic resistance Rpc in the semiconductor substrate 1020. Since an in-phase signal and a reverse-phase signal are applied to the first capacitor 1003 and the second capacitor 1004 (or vice versa), a junction between the parasitic capacitances Cpa1 and Cpa2 serves as an apparent ground (hereinafter referred to as “virtual ground”) with respect to an AC signal. As a result, the influences of the parasitic capacitance Cpc and the parasitic resistance Rpc on the resonator circuits can be reduced.
However, if the conventional differential capacitor structure were to be applied to the differential oscillation circuit 7, the parasitic capacitances Cpa1 and Cpa2 (see FIG. 20) would in effect be in parallel connection with the first variable capacitor 1005 and the second variable capacitor 1006. Moreover, since the parasitic capacitances Cpa1 and Cpa2 have fixed values, the amounts of capacitance variation in the first variable capacitor 1005 and the second variable capacitor 1006 would become smaller than their respective spec values, thus resulting in the oscillation frequency range of the differential oscillation circuit 7 being narrowed.
Meanwhile, it has also become commonplace to integrate a differential antenna element on a semiconductor. FIG. 21 is a perspective view illustrating the structure of a conventional planar differential antenna element 7001. In the planar antenna element 7001 shown in FIG. 21, two planar antenna elements 7002 and 70003, which are disposed with a predetermined interval from each other on a silicon substrate 7004 (as one example of a semiconductor substrate), outputs an in-phase signal and a reverse-phase signal, which have the same power but have a 180° phase difference between each other. As a result, the differential antenna element 7001 can receive wide-band signals.
However, when the planar antenna elements 7002 and 7003 are formed on the silicon substrate 7004, the following problems will arise: coupling occurs due to a parasitic capacitance between the wiring and the silicon substrate 7004; and a loss occurs due to the influence of the parasitic resistance on the silicon substrate 7004. As a result, the gain of the differential antenna element 7001 will be deteriorated.
FIG. 22 is a schematic diagram illustrating the structure of a balanced high-frequency device 8001 incorporating a conventional differential resonator. In FIG. 22, the balanced high-frequency device 8001 includes: an input terminal IN for receiving an input signal; a balanced device 8002 having output terminals OUT1 and OUT2 from which to output an in-phase signal and a reverse-phase signal; and a ½ wavelength resonator 8003 (as an example of a differential resonator). When receiving a signal having a predetermined frequency, the ½ wavelength resonator 8003 resonates so as to decrease an impedance of the in-phase signal component, which exists when the output terminals are viewed from the side of the balanced device 8002, so as to be lower than an impedance of the differential signal component which exists when the output terminals are viewed from the side of the balanced device 8002. Thus, the balanced high-frequency device 8001 suppresses the in-phase component, and improves the degree of balance between the output signals.
However, when the ½ wavelength resonator 8003 is formed on a silicon substrate (as an example of a semiconductor substrate), the following problems will arise: coupling occurs due to a parasitic capacitance between the ½ wavelength resonator 8003 and the silicon substrate; and a loss occurs due to the influence of the parasitic capacitance on the silicon substrate. As a result, the insertion loss of the ½ wavelength resonator 8003 will increase.